The invention discloses a dolomite-molybdenum trioxide composite catalyst for preparing acrolein by glycerol dehydration and a preparation method thereof, and the method comprises the following steps: crushing, grinding, washing and drying natural dolomite, putting into a muffle furnace, heating to 500-700 DEG C, and calcining for 4 hours to obtain calcined dolomite; ammonium molybdate tetrahydrate is dissolved in deionized water, and an ammonium molybdate tetrahydrate solution is obtained; adding the calcined dolomite into an ammonium molybdate tetrahydrate solution, stirring for 4-6 hours, dipping for 24-36 hours, drying in a drying oven, heating to 700 DEG C in a muffle furnace, and calcining for 4 hours, so as to obtain the product. The method is simple in operation method, low in price, clean, efficient and environment-friendly, the required process equipment is also very simple, the time cost can be reduced, and the method is very suitable for large-scale production; by adopting the catalyst disclosed by the invention, the selectivity of acrolein can reach more than 50%, and the catalyst has certain carbon deposition resistance, so that a better technical achievement is achieved.